I do not think that you will expose the cooling DX AHU to the weather in Winter. You can used the indirect chilled water coil with Glycol, if you need to do cooling at the roof top.
Withe current VAV model, the fresh air will flow into the zone regardless at 14°C or whatever value.
When damper is at minimum, the re-heat is turn on. When the damper is maximum, the Zone Dx cooling coil is turned on. This is the only addition, and is for a special zone. I have not added any mechanical fan in the loop.
When I said change the order of heating and cooling, I simply meant to place the heating coil in front of the cooling coil in the AHU configuration. The program would test the inlet air temperature for heating or cooling first. If the inlet air temperature is low, the cold air will always be heated before reaching the DX cooling coil. You can still turn on the cooling coil to condense out the moisture for dumidity control. The air is heated up again by HE (building ducts) or reheat heater. This is a traditional HVAC. The fan speed can be variable, if correct control algorithm is applied.
I do not hate ASHRAE. They are doing a good job in building regulations. I have not read their guide lines, because the documents are not free.
LEED certification has generated many jobs. Overloaded under sized mechanical systems have also created more jobs. They are doing well to help the economy. I am still buying LEED certified condos.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: jeannieboef@xxxxxxxxx
Date: Fri, 6 Jul 2012 20:23:00 +0000
Subject: [EnergyPlus_Support] Re: Learn from my mistakes - EMS to avoid DX coil freeze - bad implementation
To re-iterate my first statement:"Because cold climates should a) have a freeze protection heating coil to avoid air at minus deg C temperatures from entering any system, regardless."
This should not be done to stop warnings. This should be a temporary work around (until a freeze protection is properly integrated into the current models) to ensure that the dx system does not try to operate when receiving ice cold air in the case of the frost protector heater malfunctioning, or if the real system does not have this pre-heater.
In that respect I like how the chilled ceilings can be programed with condensate protection (with margin of safety) by turning off when condensation is predicted. If that is how real systems work, then that is how we should model them.
Brent, made a good point about the condenser coils...I'll have to research that one to see what the typical solution to this problem is. Dr. Li pointed out that the defrost cycle will kick in. This I think means that the unit can however no longer cool. The coils on the roof are packed in ice and snow, but the data center still needs cooling...what do you do? Add more glycol?
Just because I know how much Dr. Li hates ASHRAE and LEED I'm going to ask what ASHRAE has to say about, pre-heating coils (only since it's weekend and my 90.1 standard is on my desk in the office...I don't sleep with it under my pillow). How does one implement pre-heating coils in the case where your proposed case has no mechanical system and you're forced to model the baseline systems into your proposed model...can you use pre-heaters?
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